1. Technical Field
The embodiments herein generally relate to wireless technology, and, more particularly, to harvesting ambient radio frequency (RF) electromagnetic (EM) energy for supplying, storing, recharging or supplementing the energy necessary to power active or passive wireless electronic devices, sensors and sensor networks.
2. Description of the Related Art
The ability of actively powered, battery assisted passive (BAP) and passive wireless sensors to remotely acquire, store and/or transmit data may open up a wide variety of medical, diagnostic, quality control, safety, supply chain, logistic and security applications. For an individual, the ability to determine, prior to consumption, whether a food item contains harmful bacteria or ingredients to which the individual is allergic is highly desirable. From a population's perspective, recent national and international events have increased the need for personal or distributed systems that can monitor and detect continuously in real-time or at periodic or irregular intervals in time, chemical agents, biological agents, radiological agents and other hazards locally or over wide geographical areas.
Examples of personal handheld wireless devices that can address these issues are cellular telephones (cell phones), personal data assistants (PDAs) and portable computers or notebooks (PCs). These are pervasive electronic devices that are networked via the Internet or cellular telephone networks. Generally, the abovementioned consumer electronic devices all require local energy storage (batteries) to provide mobility and portability, and do so until their local source of energy is depleted.
A new class of sensors such as Electronic Product Code (EPC) and Radio Frequency Identification (RFID) tags that include sensing capabilities are emerging as a generally inexpensive and effective means of addressing many wireless sensor applications such as, but not exclusively, cold-supply chain, food safety, quality control, environmental safety, medical, diagnostic, electro-immunoassays, consumer goods as well as homeland security, property and personal security applications.
Purely passive sensors, when actively interrogated by an RF transceiver (reader), use this received source of EM energy to power themselves up, to acquire readings from their sensors and then rebroadcast or reflect their specific identification code and sensor readings back to the interrogator. BAP tags use their batteries to acquire and store or log data from one or more sensors, but typically do not use their batteries to enhance their RF communication abilities. Active sensors have their own built in power source that can be used to acquire and store sensor readings at any time as well as enhance the reception and transmission of RF communications. Generally, passive EPC and RFID tags equipped with one or more sensors will require a source of energy to measure and store their acquired information at times other than during active interrogation by a reader.
Because of the cost of the conventional sensors and sensor readers in particular, broad deployment of a sensor network over a large geographical area or widespread use by individuals is currently not particularly feasible. Generally, a considerable problem with large geographical deployment is that if the sensors are actively powered, their power sources need to be replaced when depleted, again adding to the cost. In addition, wear on sensors or sensor surfaces, generally requires sensors to be replaced on a regular basis further increasing the cost. In addition, imprecision in sensors generally requires cross validation to eliminate false positives, increasing the number of sensors that must be deployed for each application and thus also increasing the cost.
Accordingly, in view of the foregoing, there remains a need for low cost wireless sensors that can harvest ambient EM energy to power themselves up, acquire sensor data or recharge their power sources (batteries in the case of active sensors and batteries in BAP sensors).